An organic electroluminescence device (hereinafter also referred to as “organic EL device”) has been known, in which an organic thin film layer including a light emitting layer is disposed between an anode and a cathode, holes and electrons injected into the light emitting layer are recombined to form excitons, and the exciton energy is released as light.
Since the organic EL device is a spontaneous emitting device, it has been expected to provide, using its advantages as the spontaneous emitting device, a light emitting device having a high emission efficiency, a high image quality, a low power consumption, and a thin good design. It has been also known to make the light emitting layer into a host/dopant light emitting layer by doping a host with a light emitting material (dopant).
In the host/dopant light emitting layer, excitons can be efficiently generated from charges injected into a host material. The energy of generated excitons is transferred to the dopant, and the light emission with high efficiency from the dopant can be obtained.
To improve the performance of organic EL device, the recent study is directed also to the host/dopant system, and the search for a suitable host material and other materials for organic EL device has been continued. Patent Literatures 1 to 10 disclose various compounds wherein a carbazole structure is bonded to a non-fused or fused azine ring directly or via a linking group.
The method of forming each layer of an organic EL device is roughly classified into two, i.e., a vapor deposition method, such as a vacuum vapor deposition method and a molecular beam epitaxy method, and a coating method, such as a dipping method, a spin coating method, a casting method, a bar coating method and a roll coating method. The material to be made into a layer by a coating method is required to meet the properties not required for the material for use in a vapor deposition method, for example, the heat resistance and the solubility to a solvent. Therefore, a material useful in a vapor deposition method is not necessarily useful in a coating method. In addition, the material for use in a coating method should meet various performance requirements of organic EL device, while capable of forming a layer by a coating method.
The coating method is applicable particularly to the formation of a layer in the production of a large-sized organic EL display and lighting panel. Therefore, it has been required to develop a material for organic EL devices usable in the coating method.
Patent Literatures 1 to 8 disclose compounds in which a non-fused nitrogen-containing ring has one or more biscarbazole-containing groups or tricarbazole-containing groups. However, Patent Literature 1 to 8 fail to disclose a compound in which a fused nitrogen-containing ring has different biscarbazole-containing groups, different tricarbazole-containing groups, or both of a biscarbazole-containing group and a tricarbazole-containing group.
Patent Literature 9 discloses a compound in which a fused nitrogen-containing ring has a biscarbazole-containing group. However, a fused nitrogen-containing ring compound having two or more biscarbazole-containing groups is not disclosed therein. In addition, this document is completely silent about forming a layer by a coating method.
Patent Literature 10 teaches that a compound in which a non-fused or fused nitrogen-containing ring has more than one biscarbazole-containing group or tricarbazole-containing group is applicable to an organic EL device. It is further taught that some of the compounds are soluble in a solvent and an organic EL device is produced by forming a layer by a coating method. However, the compounds used in the production of organic EL device by forming the layer by a coating method all include the same biscarbazole-containing groups on the non-fused nitrogen-containing ring. Therefore, the properties verified in Patent Literature 10 are limited only to a compound having the same biscarbazole-containing groups on the non-fused nitrogen-containing ring, and the compound disclosed therein is still insufficient to meet the various performance requirements of an organic EL device.